Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI), para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl or T, also commonly referred to as terephthaloyl chloride) are known in the art. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586.
One issue in using DAPBI is that it has very poor solubility in a NMP/CaCl2 solvent system. In order to prepare DAPBI/paraphenylene diamine (PPD) copolymers from copolymer solutions having higher concentrations of polymer in the solvent system (i.e., above 4% weight percent polymer in the solvent system), DAPBI is typically first fully dissolved in the solvent system by heating the combination to a temperature of about 70° C. or higher followed by addition of PPD. PPD dissolves readily in the solvent. The mixture of diamines in solution, however, then has to be cooled to lower temperature (i.e. ˜5° C.) before starting polymerization with TCl. DAPBI precipitates back out during this cooling process and polymerization becomes “heterogeneous”, with the PPD remaining in solution and the DAPBI in solid state. It is believed the PPD in such a solution preferentially reacts with the TCl upon its addition versus the DAPBI in its un-dissolved state, creating highly heterogeneous block polymers, with blocks of predominantly PPD and then blocks of predominantly DAPBI.
Another issue in using DAPBI is the two amines on DAPBI are very different in reactivity and positional factors. The amine shown to the right in the below structure (the azole amine) is an order of magnitude more reactive than the amine to the left in the structure (the benzyl amine).
As a result, DAPBI/PPD copolymers prepared by a conventional polymerization process in a NMP/CaCl2 solvent system tend to have no control over the position of the monomer components. It is believed a factor in creating higher strength fibers from DAPBI/PPD copolymer is the arrangement of the comonomers along the polymer chain. In particular, controlling the distribution of PPD and DAPBI comonomers is believed to help prevent the formation of crystal solvates in sulfuric acid solution and help the alignment of polymer chains during the heat treatment of fibers made from the copolymer, resulting in fibers with better mechanical properties.